1. Field of the Invention
This invention relates to a process for manufacturing a piece made of composite material and more particularly a process and a device whose purpose is to ensure a homogeneous impregnation of a fiber preform by injection or infusion of a product that is able to form a matrix.
2. Description of the Related Art
The pieces made of a composite material comprise a matrix, for example made of resin, reinforced by fibers. According to one widely used embodiment, the fibers come in the form of a fiber preform, with one or more fold(s) that are woven or not and pre-impregnated or not.
Prior to the polymerization phase, the product that forms the matrix is to impregnate this preform homogeneously to obtain a piece that has optimal characteristics.
According to a first operating mode, referred to as injection, the product that forms the matrix, generally resin, is injected into the preform at one or more points and even over the entire surface of the preform, optionally with a diffusion medium.
According to another operating mode, referred to as infusion, the product that forms the matrix is integrated in the preform and comes in the form of, for example, one or more inserted resin film(s) or film(s) placed side by side with folds that form said preform.
To implement this impregnation, a device as described in particular in the documents US2004/0219244 or US2005/0031720 and illustrated in FIG. 1 is used.
According to this document, the preform 10 is placed in a first chamber 12 that is delimited by a substrate 14 and a first semi-sealed membrane 16, i.e., permeable to gas but sealed to the product that is able to form the matrix, whereby at least one feed point 18 is provided in said first chamber 12.
In addition, the device comprises a second chamber 20 that is delimited, on the one hand, by a second gas-tight membrane 22, and, on the other hand, by the first semi-sealed membrane 16, whereby said chamber 20 comprises at least one opening 24 for extracting the gases that are contained in said second chamber 20 and thus drawing in the gases that are present in said first chamber 12.
The intake of gases into the first chamber 12 produces the diffusion of the product that is able to form the matrix in the entire preform.
Thus, during the infusion or injection phase, the first semi-sealed membrane 16 ensures an optimum filling and degassing of the preform without drawing in the product that is able to form the matrix.
Sealing means 26 are provided to ensure the seal between the substrate 14 and the first semi-sealed membrane 16 as well as sealing means 28 between the substrate 14 and the second sealed membrane 22.
In addition, a draining fabric 30 can be placed in the second chamber so as to promote the evacuation of gases.
To obtain the smoothing of the surface of the preform that is in contact with the first semi-sealed wall 16, a smoothing plate 32 can be used and placed in the second chamber 20, inserted between the draining fabric and the first semi-sealed wall 16. Taking into account its function, this smoothing plate 32 is solid and does not comprise any through opening. It also makes it possible to homogenize the pressure forces on the preform.
According to one embodiment, the semi-sealed membrane 16 consists of a microporous fabric, whereby the small diameter of the pores makes possible the passage of gases but blocks the passage of viscous fluids such as the product that is able to form the matrix.
During the polymerization, the product that is able to form the matrix tends to solidify and to block the pores at the surface that have been in contact with the piece that is produced although the membrane made of microporous fabric is disposable.
According to one problem, whereby the microporous fabrics exist only in limited width, the large pieces have surface defects at junction zones of the bands of microporous fabrics.
According to another problem, the microporous fabrics are not very deformable; their installations on surfaces that cannot be developed can prove problematical and lead to generating folds that produce surface defects at the piece that is produced.
According to another problem, when the membrane 16 made of microporous fabric is applied on zones with small curvature radii, this tends to deform the pores and to increase their dimensions, leading to a loss of seal on the part of the product that is able to form the matrix facing said zones.
Finally, the product that forms the matrix tends to stick to the membrane 16 made of microporous fabric and makes it necessary to use a peelable film called “peel ply” that tends to generate surface defects at the piece that is produced.
The surface defects at the piece are reflected by decreased mechanical characteristics, due to the undulations of fibers, but also by decreased aerodynamic characteristics when the surface of the piece that is produced corresponds to the outside surface of an aircraft fuselage.
In addition, these surface defects do not promote the monitoring of pieces thus produced, in particular the detection of cracks and porosity, by wave reflection techniques, generally used in the aeronautical industry.